a. Field of the Invention
The present invention relates to a copying objective lens systems and, more particularly, to a copying objective lens system of four-component six-element lens configuration to be used with magnification about X1.
b. Description of the Prior Art
When copying an original of large size, for example 600 mm, with magnification of X1, brightness of the copy should be uniform over the whole field and therefore, for the lens system to be used for this purpose, aperture efficiency should be high and distortion should be minimized. Besides, as the object is flat, flatness of the image should be favourable. Moreover, to facilitate mounting and handling, the lens system should be as compact as possible.
Besides, as copying lens systems to be used with magnification about X1, it is known that the lens system arranged perfectly symmetrically in respect to the stop positioned at the center is more preferable because aberrations can then be corrected favourably. For lens systems arranged perfectly symmetrically, the entrance pupil and exit pupil of offaxial principal rays come to the same plane and, therefore, distortion and lateral chromatic aberration are not caused. To correct aberrations favourably, attention should be therefore paid only to spherical aberration, longitudinal chromatic aberration and astigmatism. For this type of lens system, coma becomes symmetrical and attention is required only to make the value to coma small. Moreover, in practice, the value of coma does not become so large when spherical aberration and astigmatism are corrected favourably. Besides, curvature of field also becomes favourable when increase of astigmatism toward the marginal portion of field and astigmatic difference are small.
However, for this type of lens systems which are commercially available at present, the F number is 9 and, therefore, the image is dark. Moreover, because of vignetting, the image becomes darker at the marginal portion of the field compared with the central portion of the field and brightness of image becomes irregular. To make brightness of image uniform, it is therefore necessary to stop down the lens system. Consequently, the exposure time becomes long and a large illuminating system is required in order to illuminate the object brightly.